Ackman, who runs $12 billion Pershing Square Capital Management, is shorting Herbalife, a multi-level marketing firm that sells nutrition products. He believes the company is a pyramid scheme and has a price target of zero.

The activist investor has pledged to donate 100% of his personal profits he makes from the short to charity.

Herbalife's stock is currently above the pre-Ackman short level.

Since December 18, the trading session before Ackman confirmed his short, Herbalife's stock is up about 1.2%. The stock touched a 52-week low of $24.24 on December 24th.